Liberalization of the power industry is the future trend of power development. Small distributed generating systems like cogeneration generating systems, solar generating systems and wind generating systems will also be integrated into the operation of the power distribution system in the future.
However, integrating distributed generating systems into the public electric power grid will affect the operation and control of the power system like the quality of the power distribution system, the possibility of stand-alone operation and the stability of the power system.
In particular, the stand-alone operation, which may happen after integrating distributed generating systems into the public electric power grid constitutes a large threat to the safety and stability of the power system. FIG. 1 is a power distribution architecture diagram of a conventional distributed generating system. In this power distribution mode, when a branch breaker 101 of a public electric power grid 100 is open-circuited, distributed generating systems DG1, DG2 and DG3 remain in the discharging state, i.e., forming independent power islands not controlled by the power company.
The problem is that distributed generating systems usually are private, and are not directly controlled by the power company. When the power company cuts off the electricity for any reason, each distributed generating system can't detect this situation and thus can't immediately separate from the public electric power grid 100, causing stand-alone operation of part of the power distribution system. If it is not carefully planned in advance, the coverage of this incident may expand to result in great damage.
The stand-alone operation of distributed generating systems usually occur when the total generating capacity is very close to the total load of the power distribution system. This phenomenon may cause the following harmful results.
1. Because the independent power island still provides power and hasn't been detected, maintenance men may be careless and compromise their labor safety.
2. The automatic restoration and shut down procedure of the power distribution system will be affected.
3. During the stand-alone operation, the voltage and frequency are unstable. That is, if the generating capacity of the distributed generating systems in the independent power island is lower than the total dissipated power, the system voltage and frequency will drop.
Therefore, the power specifications of advanced nations have prescribed that when the public electric power grid malfunctions or cuts off electricity, distributed generating systems ought to separate immediately from the public electric power grid to avoid an abnormal charging state of the public electric power grid.
However, for a conventionally used voltage/frequency protection relay, the variation of voltage and frequency on the power distribution system is indistinct because the distributed generating systems still provide power when the public electric power grid cuts off electricity. If the power distribution system has a motor load, the variation of voltage and frequency will be even more indistinct and insufficient to trigger the conventional protection relay, making a branch breaker unable to separate from the public electric power grid.
Recently, parameter monitoring techniques of the power system have been developed for stand-alone operation of distributed generating systems. Protection relays for stand-alone operation have been proposed for solving the dead angle problem of detection of the conventional voltage/frequency relays. These parameter monitoring techniques of the power system can be classified into passive detection methods and active detection methods.
Passive detection methods (e.g., phase change detection and frequency shift detection) are bases on system parameter variations to determine the occurrence of stand-alone operation. These methods have the advantage of low installation cost, but they can't detect stand-alone operation for the difference of supply and demand of the power distribution system in independent power island.
Active detection methods detect frequency variation and real power variation by adding a small perturbation in the output terminal of the generating system. When the system is normal, the public electric power grid is robust enough so that this small perturbation won't cause any problem. On the contrary, once the public electric power grid cuts off electricity, this small perturbation will cause a large variation of frequency or voltage to help the distributed generation system separate from the public electric power grid immediately. However, if there are many distributed generating systems integrated into the public electric power grid, it will be more difficult to grasp the accuracy of the added small perturbation, or the power supply of the system will even be unstable.
Accordingly, the present invention aims to propose a system for detecting stand-alone operation to detect accurately and quickly the occurrence of stand-alone operation.